1. Field of the Invention
This invention relates to a zoom lens system, and more particularly to a compact zoom lens system suitable for a compact camera such as a video camera.
2. Description of the Prior Art
In recent years, a body of a camera such as a video camera becomes more compact in size and in weight and becomes more cheap, since packaging and integration rate of electric circuits occupied therein have improved. As a result, a volume, weight, and cost of the a lens system occupying in the video camera is increasing relatively, thus it has been a strong demand to make the lens system compact and to reduce the cost thereof.
On the other hand, lens system occupying in video camera is required to have a larger aperture ratio for avoiding the decrease of light intensity on higher image height region caused by compactness of image pick-up device, and also to have a higher performance of a higher integration and a higher resolution.
In the present invention, the optical performance required to the zoom lens system is defined as a zooming ratio of 3 to 8, and an F-number of 1.4 to 1.6. Many type of zoom lens system fulfilling the above-defined optical performance have already been proposed, but almost all proposals require much number of lens elements reaching thirteen (13) or fifteen (15) elements. Therefore, such zoom lens system is difficuit to reduce the cost.
Thus, the other type is proposed in which the number of lens elements required can be decreased by applying at least an aspheric surface, as proposed by Japanese Laid-Open Patent Application Nos. 56-147113 and 61-110112. The former proposes a 4-group zoom lens system comprising from the object side, a first positive lens group, a second negative lens group, a third negative lens group, and a fourth positive lens group, consisting of ten (10) lens elements with applying three aspheric surfaces. Such a zoom lens system, however, cannot fulfills the above-defined optical performance since it has a small zooming ratio of three (3). The latter also proposes a 4-group zoom lens system comprising positive-negative-negative-positive lens groups from the object side and consisting of eight (8) lens elements with applying four aspheric surfaces. The latter proposal cannot fulfills the sufficient optical performance required.
The assignee of the present invention proposes, by Japanese Laid-Open Patent Application Nos. 1-46716 and 1-46717, 3-group zoom lens system comprising from the object side, a first negative lens group, a second positive lens group and a third positive lens group. Such a proposed zoom lens system consists of nine (9) or ten (10) lens elements and has an F-number of 1.2 to 0.9 without applying any aspheric surface. However, the zoom lens system becomes large and the aberrations thereof is deteriorated if its zooming ratio is set to six (6) to eight (8) defined in the present invention.
The assignee of the present invention also proposes, by Japanese Laid-Open Patent Application No. 1-74517, 3-group zoom lens system comprising from the object side, a first positive lens group consisting of only two lens elements, a second negative lens group consisting of only two lens elements, and a third positive lens group. Such a zoom lens system consists of only eight (8) lens elements without applying aspheric surface, and therefore is compact. However, it has a small zooming ratio of 2 to 3 while having a sufficient F-number of 1.6 to 1.4, and is difficult to extend the zooming ratio to about 6.